(a) Field of the Invention:
The present invention relates to an electrophotography, and more particularly it pertains to a multi-layer photoconductive structure having a flexibility and a sensitivity to a light ray spectrum including red color.
(b) Description of the Prior Art:
In the art of electrophotography, a photoconductivelayer-containing plate is first given a uniform electrostatic charge by, for example, moving the plate between two corona-discharging devices in darkness. The resulting sensitized plate is exposed to light rays through a positive or negative transparency which is to be reproduced, so that the electric charges on those areas of the photoconductive layer exposed to light rays are caused to dissipate, leaving behind a latent electrostatic image in the areas of said layer not illuminated. This latent electrostatic image may be developed by depositing, onto the resulting surface of the photoconductive layer, a toner which is composed of finely pulverized particles of a mixture of a resin and black carbon powder to produce a visible image. This visible image may then be transferred onto a surface of a sheet of paper, and the resulting image-bearing sheet is heated to melt and solidify the resin. In this way, a more or less permanent image can be obtained. This concept of electrophotography was originally proposed by C. F. Carlson.
As an alternative method, the step of transferring the visible image onto a surface of a sheet of paper may be omitted by using, instead of the above-mentioned plate, a sheet of paper or other appropriate supporting sheet coated, on its surface, with a photoconductive material. This sheet itself serves as a copying sheet, and a visible image is produced directly on this sheet. This technique is known as "electro-fax".
The photoconductive-layer-carrying plate used in Carlson's method is required to possess the following principal properties. They are:
1. a high electric resistivity in darkness, and
2. a sufficient dissipation of this resistivity when exposed to light rays. These properties may be explained in more concrete terms as follows. Basically speaking, the plate is required to have a capability of being electrically charged up quickly up to a great amount of charge and to be capable of sufficiently retaining the charged electricity in darkness; and furthermore, the charged plate has to be capable of quickly dissipating the charged electricity down to a sufficiently low level of potential by its exposure to light rays, and the plate needs to have a good sensitivity to a wide range of spectrum of light rays. The electrophotographic plate for actual use on a copying machine which is designed to make copies repeatedly is required to have additional properties such as a high resistance to fatigue caused by the repetition of exposure to light rays as well as by the repetition of electrical charging, a high mechanical strength, a great deal of stability against ambient conditions, innocuousness for the body of the human being who handles the plate, easy to manufacture at low cost, and like requirements. It is often the case that these additional properties become important. From the foregoing viewpoints, photoconductive materials such as Se, ZnO, CdS and Polyvinyl-Carbazole (PVCz) have been heretofore put to practical use as a predominant component of the photoconductive layer. So long as the discussion on the non-mechanical printing technique is limited to Carlson's method among all those known techniques, however, a photoconductive layer consisting predominantly of Se may be safely said as being the best. However, even such Se-based photoconductive layer has still much to be improved with respect to electrostatic and mechanical properties, and efforts are being made to develop a new method of producing improved photoconductive materials to obtain an electrophotographic plate having improved electrostatic and mechanical properties.
Generally, amorphous selenium is employed for the production of a photoconductive layer containing selenium, because amorphous selenium has a higher resistivity in darkness and has an enhanced ability to retain charged electricity than does crystal selenium. However, amorphous selenium has a sensitivity to only a limited short wavelength portion of the electromagnetic spectrum and, therefore, it is not suitable for the reproduction of colored originals. In order to alleviate this disadvantage, a composite comprising finely pulverized crystal selenium dispersed in a matrix of amorphous selenium has been employed instead of amorphous selenium alone for the manufacture of a photoconductive layer so as to impart to the layer the good sensitivity of the crystal selenium to longer wavelengths of light rays. Notwithstanding such effort, the sensitivity of the resulting composite has been found to be still unsatisfactory because of essential lack of the sensitivity to that portion of electromagnetic spectrum around 6200A. Moreover, amorphous selenium is unstable in its morphology and has a tendency to transform into a more stable form as crystal selenium. As stated above, selenium, in its crystal form, has a low resistivity and does not retain charged electricity for a sufficient length of time. On the other hand, amorphous selenium is frangible and lacks flexibility, so that a layer thereof coated on a substrate tends to come off from the substrate and to fracture from a slight bending. Thus, amorphous selenium has the disadvantage that the available configuration of the electrophotographic plate containing the amorphous selenium photoconductive layer is limited. In addition, there is an increasing demand for a higher speed printing. To meet this demand, there is needed the provision of an electrophotographic plate which has a much higher sensitivity over a wide range of wavelengths in the electromagnetic spectrum and which can respond at a higher speed to charge-up and exposure operations. A number of proposals have been made to increase the speed of response and to make selenium material sensitive up to longer wavelengths of the spectrum by doping the selenium material with an impurity. One of known such methods employs doping of tellurium (Te). Other than this, Japanese Patent Publication No. 42- 13233 discloses the use of appropriate amounts of As and I as dopants, Japanese Patent Publication No. 44-12670 showns the employment of halogens, Japanese Publication No. 44-23556 discloses the employment of As, Br, and Cl, Japanese Publication No. 46-15478 discloses the employment of Sb and Japanese Publication No. 46-42679 discloses the employment of Sb-As, in appropriate amount, respectively. However, these methods are invariably more or less unsatisfactory. For example, "As" is toxic and must be handled with great care during the process of manufacture. Therefore, the inclusion of As in selenium material is not desirable. The experiments conducted by the inventors show that the addition of halogen leads to limiting the sensitivity of selenium to regions of shorter electromagnetic wavelengths. Thus, the halogen-doped selenium has hardly any sensitivity to red color, and moreover, it has little flexibility. Therefore, the photoconductive layer formed with halogen-doped selenium is practically without the advantage which selenium alone possesses. Antimony (Sb) has a marked tendency to segregate in a source melt during an ordinary vacuum evaporation-deposition process, making it difficult and unpractical to produce a uniform distribution of antimony at a desired concentration thereof throughout the photoconductive layer. At any rate, the above dopants have been employed usually to form a photoconductive layer on a substrate which is in the form of a plate or a cylindrical drum, because selenium lacks flexibility. An attempt to impart flexibility to selenium by doping it with sulfur (S) has been proposed in Japanese Patent Publication No. 43-29431. According to the results of the experiments conducted by the inventors, it has been found that as the concentration of sulfur in selenium material is elevated, the material becomes more flexible, but its response to optical exposure becomes slow. Besides, the wavelength range of spectrum to which the material is sensitive becomes limited to wavelengths shorter than the wavelengths to which the amorphous selenium having, dispersed therein, fine particles of crystal selenium is sensitive. Thus, it has been found that the sulfur-doped selenium material not only has no sensitivity to red color, but also exhibits a very high residual potential, and that in particular it has a very poor durability against repetition of a printing cycle. The slow response to optical exposure may be permissible in such a case wherein a high speed reproduction or copying is not required. However, an electrophotographic plate having a high residual potential gives rise to the formation of background which is so-called "fog" or "blurring" and brings about a poor contrast in the copy reproduced. Therefore, the sulfur-doped selenium material is not practical. The fact that such material is not sensitive at all to red color in the spectrum and the fact that the wavelengths to which it is sensiting become shorter with an elevation of the doping density of sulfur may be demonstrated also from the aspect of transmissivity of light rays exhibited by the material having a sulfur concentration as will be described in detail layer. As other techniques not using a dopant, Japanese Patent Publication No. 49-6228 discloses a method wherein selenium (Se) and arsenic (As) are dispersed in a solution of chlorinated rubber, and this dispersion is melted and quenched. Similarly, Japanese Patent Publication No. 50-10733 discloses to the art that red hexagonal selenium pigment is dispersed in a binder, and the dispersion is melted and quenched. Japanese Patent Publication No. 50-34414 discloses to the art that powdery selenium is dispersed in phthalocyanine, and the dispersion is melted and quenched. These mixtures shown in the above-mentioned known methods invariably have inferior properties as compared to those of amophous selenium. The composite material comprising selenium pigment dispersed in the binder has a poor resistance to solvent and, hence, there is the necessity for careful selection of a wet developer. As discussed above, the prior techniques are in general more or less unsatisfactory, and thus there has been a need for an improved photoconductive material or structure having desirable electrostatic and mechanical properties.